1. Fie1d of the Invention
This invention relates to particulate separation of material from an aqueous slurry by a dissolved air flotation system, and more particularly to a system wherein flotation air is introduced by flowing pressurized air through an eductor and venturi to aspirate a low volume flow rate of water thereinto.
2. Description of the Prior Art
Commercially valuable minerals, for example metal sulfides, apatitic phosphates and the like, are commonly found in nature mixed with relatively large quantities of gangue materials, and as a consequence it is usually necessary to beneficiate the ores in order to concentrate the mineral content thereof. Mixtures of finely divided mineral particles and finely divided gangue particles can be separated and a mineral concentrate obtained therefrom by well known froth flotation techniques. Broadly speaking, froth flotation involves conditioning an aqueous slurry or pulp of the mixture of mineral and gaugue particles with one or more flotation reagents which will promote flotation of either the mineral or the gangue constituents of the pulp when the pulp is aerated. The conditioned pulp is aerated by introducing into the pulp a plurality of minute air bubbles which tend to become attached either to the mineral particles or the gangue particles of the pulp, thereby causing one category of these particles, a float fraction, to rise to the surface of the body of pulp and form thereat a froth which overflows or is withdrawn from the flotation apparatus. The other category of particles, a non-float fraction, tends to gravitate downwardly through the aqueous pulp, and it may be withdrawn at an underflow outlet from the flotation apparatus. Typical examples of such flotation apparatus for accomplishing the foregoing are disclosed in U.S. Pat. Nos. 2,753,045; 2,758,714; 3,298,519; 3,371,779; 4,287,054 and 4,394,258.
In such apparatus, the conditioned pulp is introduced into a flotation compartment containing a relatively quiescent body of aqueous pulp and aerated water is introduced into the lower portion of the flotation compartment through orifices formed in the bottom wall of the flotation compartment. An overflow fraction containing floated particles of the pulp is withdrawn from the top of the body of aqueous pulp and an underflow or non-float fraction containing non-floated particles of the pulp is withdrawn from the pulp in the lower portion of the flotation compartment.
In several of the heretofore known systems, the aerated water is produced by first introducing, by injection, a frother or surfactant into the water, which mixture is then passed through an eductor wherein air is aspirated into the water. In order to obtain a proper degree of aeration of the water, a high flow rate of water, typically in excess of 1,000 gallons per minute, must be passed through the eductor. While recirculation systems have been devised to minimize the amount of "new" water added to the system, a significant expenditure in energy is required to move such large quantities of water.
A further problem encountered arises from the difference between the concentrations of solid particles present in slurries of different minerals. Phosphates, for example, do not typically require extensive grinding in order to liberate the desired mineral components of the pulp. As a result, the aqueous slurry or pulp fed to the flotation apparatus typically consists of approximately seventy-five percent (75%) solids and twenty-five percent (25%) water. Sulfides, on the other hand, approach the obverse extreme and typically require extensive beneficiation through grinding the material to a very fine state in order to gain liberation of the desired minerals from the gangue. The addition of water throughout the sorting, grinding and classifying stages of the beneficiation process provides a resulting aqueous slurry to the flotation device comprising approximately ten percent (10%) solid matter and ninety percent (90%) water. Thus, the addition of significant additional amounts of water through the introduction of the aerated water appears counter-productive in that significant amounts of the finely ground valuable minerals may avoid capture by the aeration bubbles and remain suspended within the liquid component of the slurry. If a recirculation system is utilized, much of the finely ground material may be passed through the recirculation system which may cause silting of the recirculation system or loss of a significant quantity of finely ground valuable minerals or both. Ideally, to avoid loss of such valuable minerals, additional air bubbles should be introduced into the aerated water. This in turn has heretofore required the introduction of still greater additional amounts of water to the system.